The goal of this project is to characterize the acute toxicity of trimethylsilyldiazomethane (TMSD). TMSD is a methylating agent used in synthetic chemistry. It is marketed as a safe replacement for diazomethane, primarily due to a lower explosive potential. However, two chemists died of acute pulmonary edema while working with TMSD in the laboratory. NTP is performing toxicity studies in order to further examine the potential for TMSD to induce pulmonary toxicity. Laboratory pathology/reporting are in progress. Keywords: toxicity, inhalation, trimethylsilyldiazomethane